1. Field
The presently disclosed embodiments relate generally to wireless communications, and more specifically to a novel and improved method of diversity searching and demodulator assignment in receivers with multiple receive chains.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, fax, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM.” Another specific CDMA system is disclosed in U.S. patent application Ser. No. 08/963,386, now U.S. Pat. No. 6,574,211, issued Jun. 3, 2003, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” (hereinafter, the HDR system). These patents and patent application are assigned to the assignee of the present invention and incorporated herein by reference.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the “TIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the IS-98 standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (4) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (5) some other standards. These standards are incorporated herein by reference. A system that implements the High Rate Packet Data specification of the cdma2000 standard is referred to herein as a high data rate (HDR) system. The HDR system is documented in TIA/EIA-IS-856, “CDMA2000 High Rate Packet Data Air Interface Specification,” and incorporated herein by reference. Proposed wireless systems also provide a combination of HDR and low data rate services (such as voice and fax services) using a single air interface.
CDMA receivers commonly employ RAKE receivers, described in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the present invention and incorporated herein by reference. A rake receiver is typically made up of one or more searchers for locating direct and multi-path pilots from neighboring base stations, and two or more multi-path demodulator fingers for receiving and combining information signals from those base stations. Searchers are described in U.S. patent application Ser. No. 09/892,280, now U.S. Publication No. 2001-0046205, published Nov. 29, 2001, entitled “MULTI-PATH SEARCH PROCESSOR FOR SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS,” filed Jun. 26, 2001, and U.S. Pat. No. 6,363,108, entitled “PROGRAMMABLE MATCHED FILTER SEARCHER,” issued Mar. 26, 2002, both assigned to the assignee of the present invention and incorporated herein by reference. The rake receiver processes a modulated signal that has been transmitted on the forward or reverse link using the searcher element and finger processors. The searcher element searches for strong instances of the received signal known as multi-paths. The finger processors are assigned to process the strongest multi-paths to generate demodulated symbols for those multi-paths. The rake receiver then combines the demodulated symbols from all assigned finger processors to generate recovered symbols that are estimates of the transmitted data. The rake receiver efficiently combines energy received via multiple signal paths.
Inherent in the design of direct sequence CDMA systems is the requirement that a receiver must align its Pseudorandom Noise (PN) sequences to the pilot PN of the base station. A base station distinguishes itself from other base stations by inserting a unique time offset in the generation of its PN sequences. In IS-95 systems, all base stations are offset by an integer multiple of 64 chips. A subscriber unit communicates with a base station by assigning at least one demodulator finger to that base station. An assigned demodulator finger must insert the appropriate offset into its PN sequence in order to communicate with that base station. It is also possible to differentiate base stations by using unique PN sequences for each base station rather than offsets of the same PN sequence. In this case, a demodulator finger would adjust its PN generator to produce the appropriate PN sequence for the base station to which it is assigned.
To improve the quality of wireless transmissions, communication systems often employ multiple radiating antenna elements at the transmitter, or diversity transmission, to communicate information to a receiver. Multiple antennas are desirable, as wireless communication systems tend to be interference-limited, and the use of multiple antenna elements reduces inter-symbol and co-channel interference introduced during modulation and transmission of radio signals, enhancing the quality of communications. Further, the use of multiple element antenna arrays at both the transmitter and receiver enhances the capacity of multiple-access communication systems. Multi-path signals may be generated at a receiver by diversity transmission or as a result of dispersion of the channel during transmission.
Thus, at a receiving destination, more than one receiver chain may be needed to process the multi-path received signals. A diversity receiver may have multiple antennas. A receiver chain for signals received at each antenna may be necessary. Therefore, multiple receiver chains may be necessary to exploit the multi-path signals received at multiple receive antennas. Receivers equipped with multiple receive chains are able to enhance their reception with improved interference cancellation and capitalization of independent fading of the receive chains, but complexity is created in the searching and assignment of the received multi-paths to the demodulation fingers of the RAKE receiver. Finger limitation becomes an important issue for diversity receivers, as compared to single element receivers, because the number of possible paths produced by each search of each active set element is a multiplied by the number of antenna elements, complicating the task of selecting the best paths for signal combining.
Thus, there is a need in the art for maximizing reception of multi-path signals by emphasizing the diversity of receive chains using optimal search and triage operations for diversity receivers.